The invention is a process for the fractional distillation of naphtha or gasoline boiling range hydrocarbon fraction. This fractionation is performed upstream of processing units designed to treat an olefin-containing overhead stream and a heavier hydrocarbon bottoms stream. The invention specifically relates to the use of a dividing wall distillation column to separate a naphtha upstream of desulfurization units.
The naphtha boiling range hydrocarbons sold commercially as gasoline are normally a blend of several streams produced in a petroleum refinery. These include reformates and alkylates which are relatively sulfur free because of upstream refining. Another major source of the naphtha boiling range hydrocarbons are processing units which do not receive a highly desulfurized feed. These include hydrocracking units, coking units and fluidized catalytic cracking (FCC) process units. The naphtha boiling hydrocarbon product streams produced by these units will contain sulfur in the form of several molecular forms including mercaptans, sulfides, disulfides, thiophenes and benzothiophenes.
Some form of sulfur removal is normally applied to these sulfur-containing hydrocarbon streams or to a blend of them to reduce the sulfur level of the final gasoline product. Increased environmental concerns are resulting in a worldwide lowering of the allowable level of sulfur in gasoline, and it has become necessary to find ways to remove increased amounts of sulfur from these hydrocarbon streams. This is especially true in the case of the full boiling range naphtha recovered from an FCC process, which often comprises a large fraction of the available gasoline pool and contains a significant amount of sulfur.
The removal of the sulfur is complicated by the various forms that it takes and by the fact that hydrotreating, one of the predominant desulfurization technologies, also hydrogenates olefins present in these streams. Paraffins tend to have lower octane numbers than the corresponding olefin and hydrogenation therefore lowers the octane number of the naphtha fraction. As described below, various methods have been developed to remove sulfur compounds from naphtha boiling range hydrocarbons. There remains however a further need for improvement in desulfurization technology which allows attaining very low sulfur levels without reducing the octane number of the fraction being treated.
It has been recognized in the art that the sulfur containing compounds in a cracked gasoline fraction tend to be concentrated in the higher boiling, or heavier, portion of the gasoline. U.S. Pat. No. 3,957,625 issued to B. A Orkin discloses this and uses it to advantage by fractionating the cracked gasoline and then hydrotreating only the heavier fraction, thus avoiding hydrogenation of the lower boiling olefins. A similar approach is shown in FIG. 1 of a paper entitled Novel Process for FCC Gasoline Desulfurization and Benzene Reduction to Meet Clean Fuels Requirements presented at the National Petrochemical and Refiners Association 2000 annual meeting March 26-28 in San Antonio, Tex.
A paper entitled Removal of Sulfur From Light FCC Gasoline, also presented at the National Petrochemical and Refiners Association 2000 annual meeting March 26-28 in San Antonio, Tex. discloses that the sulfur compounds in the initial boiling point range of light FCC gasoline are predominantly mercaptans and that these mercaptans can be removed by extraction into a caustic stream. The paper also points out that as the boiling point of the gasoline increases thiophenic, sulfur starts to appear in the gasoline. As thiophenes are not extractable by the caustic it is desirable to set an endpoint to the fraction being treated by extraction which is low enough to exclude thiophenes. The thiophenes therefore remain in the heavy fraction, which is hydrotreated for desulfurization.
U.S. Pat. No. 5,582,714 -A1 issued to P. Forte describes the problem of sulfur being present in FCC gasoline and recognizes that hydrotreatment of this stream will result in loss of octane by saturation of olefinic hydrocarbons. The patent teaches the use of liquid-liquid extraction of sulfur compounds into a solvent.
U.S. Pat. No. 6,228,254 B1 also addresses the problem of sulfur in gasoline fraction but presents a two-step solution comprising hydrotreating followed by adsorption or liquid extraction with an aqueous stream. The dividing wall or Petlyuk configuration for fractionation columns was initially introduced some 50 years ago by Petlyuk et al. Dividing wall columns have been employed for the separation of hydrocarbon mixtures as evidenced by the disclosure of U.S. Pat. No. 2,471,134 issued to R. O. Wright. Recently the use of dividing wall columns has begun to expand because of the greater recognition that in certain situations dividing wall columns can provide benefits above those of conventional fractionation columns. For instance, a commercialization of a fractionation column employing this technique is described in the article appearing at page s14 of a supplement to The Chemical Engineer, Aug. 27, 1992.
U.S. Pat. No. 2,471,134 issued to R. O. Wright illustrates a dividing wall fractionation column having a partition or dividing wall 20 dividing the trayed column into two parallel vapor-liquid contacting chambers. A similar but more detailed disclosure of a dividing wall fractionation column is provided by U.S. Pat. No. 4,230,533 issued to V. A. Giroux. Dividing wall columns are closely related to a different type of column referred to as a partitioned distillation column such as illustrated in U.S. Pat. No. 5,755,933 issued to Thomas P. Ognisty et al. A partitioned distillation column differs from a dividing wall column in that the vertical dividing wall is positioned such that it contacts one end of the column. Thus only one terminal portion of the column is divided into the two parallel contacting sections. In this manner two overhead products or two bottom products may be removed from a single column. A dividing wall column produces an intermediate boiling fraction.
It has been discovered that a significant improvement can be achieved in the overall performance of a complex which first fractionates and then desulfurizes the resultant fractions of a full boiling range gasoline by employing a dividing wall column to perform the fractionation. Such a column overcomes or at least reduces problems resulting from the tendency of the thiophenes to form azeotropes with the olefinic hydrocarbons.
A first embodiment of the invention may be characterized as a process for the desulfurization of a naphtha boiling range feed stock comprising sulfur compounds, which process comprises passing the feedstock into a dividing wall column and separating the feed stock into a light fraction comprising mercaptans, olefinic hydrocarbons and paraffinic hydrocarbons having less than six carbon atoms, an intermediate fraction comprising C6 and C7 olefinic hydrocarbons and thiophene, and a heavy fraction which comprises sulfur compounds and hydrocarbons containing more than seven carbon atoms per molecule; passing the light fraction into an extractive first treating zone in which mercaptans are removed by contact with an aqueous alkaline solution to form a first treated stream; treating the intermediate fraction in a second treating zone, in which thiophenes are removed, to form a second treated stream; treating the heavy fraction in a third treating zone, in which sulfur containing compounds are subjected to hydrotreating, to forma third treated stream; and, recombining the first, second and third streams to form a low sulfur gasoline blending component stream.